It is very difficult, if not impossible, to understand
fully what electric charge, q, is.
For SAT II Physics, you need only remember the old phrase: opposites
attract. Protons carry a positive charge and electrons carry a negative
charge, so you can just remember these three simple rules:
- Two positive charges will repel one another.
- Two negative charges will repel one another.
- A positive charge and a negative charge will attract one
The amount of positive charge in a proton is equal to
the amount of negative charge in an electron, so an atom with an
equal number of protons and electrons is electrically neutral, since
the positive and negative charges balance out. Our focus will be
on those cases when electrons are liberated from their atoms so
that the atom is left with a net positive charge and the electron
carries a net negative charge somewhere else.
Conservation of Charge
The SI unit of charge is the coulomb
The smallest unit of charge, e
charge carried by a proton or an electron—is approximately
C. The conservation of charge
first put forward by Benjamin Franklin—tells us that charge can be
neither created nor destroyed. The conservation of charge is much
like the conservation of energy: the net charge in the universe
is a constant, but charge, like energy, can be transferred from
one place to another, so that a given system experiences a net gain
or loss of charge. Two common examples of charge being transferred
from one place to another are:
Rubbing a rubber rod with a piece of wool: The
rod will pull the electrons off the wool, so that the rubber rod
will end up with a net negative charge and the wool will have a
net positive charge. You’ve probably experienced the “shocking”
effects of rubbing rubber-soled shoes on a wool carpet.
a glass rod with a piece of silk: The silk will pull the
electrons off the glass, so that the glass rod will end up with
a net positive charge and the silk will have a net negative charge.
Remember, net charge is always conserved: the positive
charge of the wool or glass rod will balance out the negative charge
of the rubber rod or silk.
The electroscope is a device commonly used—and
sometimes included on SAT II Physics—to demonstrate how electric
charge works. It consists of a metal bulb connected to a rod, which
in turn is connected to two thin leaves of metal contained within
an evacuated glass chamber. When a negatively charged object is
brought close to the metal bulb, the electrons in the bulb are repelled
by the charge in the object and move down the rod to the two thin
leaves. As a result, the bulb at the top takes on a positive charge
and the two leaves take on a negative charge. The two metal leaves
then push apart, as they are both negatively charged, and repel
When a positively charged object approaches the metal
bulb, the exact opposite happens, but with the same result. Electrons
are drawn up toward the bulb, so that the bulb takes on a negative
charge and the metal leaves have a positive charge. Because both
leaves still have the same charge, they will still push apart.